Thank you for your question.
Wow that's a tough one, we have an entire page on which fuel is best and don't really get to the definitive answer. https://www.directstoves.com/knowledge-base/home/guide-to-solid-fuels.html
As you are looking at a Defra approved stove, I will assume you want to use it in a Smoke Control Area.
You will only have the choice of wood, anthracite or manufactured smokeless coal. Of these, anthracite has the highest calorific value (9.2kWh/Kg) but is the most expensive at 41p/Kg.
Many people are buying stoves as much for the way they look and they love the impressive flame patterns given from wood over those from coal.
Hardwood is much better than softwood as it has a higher calorific value and is much denser so takes up less storage space, despite it's higher price.
I would advise you to experiment and see which you prefer, and definitely get the opinion of your chimney sweep - he's the one that sees all the problems when people use the wrong fuel.
Hope this helps?